Generally, an image sensor is a semiconductor device for converting an optical image into an electric signal. There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices, hybrid focal plane arrays, etc. The various types of image sensors may be broadly categorized as charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) image sensors.
In recent years, there has also been increased interest in applying CMOS active pixel sensors for night vision applications, such as night vision sensors used by soldiers. The night vision band of electromagnetic radiation corresponds to wavelengths in the range of 600 nm to 1000 nm.
Current night vision sensors employ intensifier tube technology. Intensifier tubes use photo cathodes having quantum efficiency (QE) of about 30-40 percent in the night vision band. Dark current shot noise is negligible over the military temperature range. Intensifier tubes are capable of producing useful images with overcast starlight illumination.
Reasons for replacing intensifier tubes include their large size and high cost. In contrast, CMOS imaging devices are of generally low cost and small size, have direct electronic output, and have a potentially higher mean time between failures (MTF).
Unfortunately, conventional CMOS imagers cannot match the low light performance of intensifier tubes because room temperature dark current is too high for devices thick enough to have high near infra-red (IR) quantum efficient (QE) needed for night vision.
To achieve overcast starlight operation without cooling, a CMOS imaging device should have at least the following properties (or better): (1) a total output noise<1 erms, (2) high QE in the 600-1000 nm range; and (3) dark current<1 e/pixel/frame up to 60 degrees Celsius. Read noise of<1 erms is presently attainable with conventional CMOS imaging technology. High near IR QE may be attainable using thick silicon to provide adequate absorption of long wavelength light. Typically, silicon thickness needs to be in the range of 15 um to 25 um. Pixels are generally 4 um to 8 um square. U.S. Patent Application publication No. US 2007/0108371 (hereinafter “the '371 application”) discloses how to achieve the needed dark current level with a relatively thin effective photon absorption and dark current generating region. This approach results in lower QE in the 600-1000 nm wavelength range than is possible with thicker silicon absorption in a CMOS pixel.
FIGS. 1 and 2 show an n-channel charge transfer pixel and a p-channel charge transfer pixel as disclosed in the '371 application, respectively. A lower dark current is asserted in the '371 application for the p-channel pixel. This is of interest for night vision applications. The '371 application describes a p-channel process which reduces many sources of dark current but not bulk dark current. Unfortunately, the lowest possible dark current limit results from bulk silicon. The '371 application claims to have dark current sufficiently low for uncooled night vision use. Unfortunately, but the absorption region for photons in such devices is too thin to have high QE at near IR wavelengths needed for night vision. The thickness of the imager is limited by the depth of an n-well implant. If the n-well were to be formed in another way so that it may be deeper, bulk dark current would increase because of the increase in silicon volume.
U.S. Pat. No. 6,433,326 (hereinafter “the '326 patent”) asserts that dark current reduction may be achieved by minimizing detector area with respect to pixel pitch and specific readout for a CMOS/CCD hybrid process imager. In the '326 patent, a detector is made as small as possible and surrounded by a guard ring to remove excess dark current and a microlens array is used to increase fill factor. Light from an objective lens is focused to a small spot by the microlens array on each detector. Unfortunately, there is no discussion of silicon volume in the '326 patent. Therefore, the bulk silicon dark current issue remains.
Accordingly, what would be desirable, but has not yet been provided, is CMOS active pixel sensor design that reduces silicon volume per pixel while still providing efficient absorption of light in the wavelength range from 600 to 1000 nm that also reduces bulk dark current.